1. Field of the Invention
This invention relates to instrumentation to detect the presence of, or to measure the concentration of, a gas or pollutant in a gaseous environment.
2. Background of the Invention
The detection of the presence of, and frequently also the measurement of the concentration of, various gases, environmental pollutants, and toxic gases, is of increasing importance. While the presence and concentration of these can usually be accomplished by the use of conventional sampling and analytical techniques, many of the situations which they represent require very rapid accomplishment, and rugged and reliable devices for the purpose. It is no longer suitable occasionally to sample stack gas or ambient air and then in what was once a reasonable time to read out what the conditions were. Frequently these situations if not corrected can result in costly waste of fuel, pollution of the atmosphere which may give rise to penalties, or to the unwarned presence or emission of toxics.
As one example, operators of combustion devices such as boilers are well aware that continuous detection and measurement of gases produced in minor quantities such as carbon monoxide, and responsive control of the processes which produce them, can result in dramatically improved fuel efficiency. In such installations excess air was formerly widely used in combustion processes on the assumption that a lean mixture would assure more complete combustion of fuel. However, as combustion processes became better understood, it also became apparent that the use of excess air was wasteful, because among other things it required the flame to heat excess gas, enabled the formation of SO3 instead of merely SO2, encouraged the formation of NO, created sulfate emissions, and in some cases even increased smoke formation by shortening the flame length. Combustion operations using low excess air improve all of the above situations, but the control must be accurate, and be quickly responsive in order to insure complete combustion while avoiding uneconomical operations and the formation of excessive pollutants. The concentration of carbon monoxide produced by a combustion process turns out to be a good measure of the average combustion quality, i.e., nearness to stoichiometric conditions. For example, no CO means too much air, while high CO means not enough air.
With the realization that controls based on the concentration of some minor component of a gas stream can lead to an optimized combustion function, serious development of suitable instrumentation was undertaken, especially instrumentation for measuring the concentration of carbon monoxide in a gas stream. Of course, measuring techniques and instruments had long existed for this purpose, but frequently they relied on sampling techniques which were too slow to provide useful data for on-line adjustment of combustion parameters, or not reliable enough for continuous duty.
The increased stringency of government regulations relating to power plant emissions has long been a prod for the development of in-situ gas analyzers, and several types of such analyzers have been installed in hundreds of power plants in recent years. Some utilize the technique known as xe2x80x9cgas filter correlationxe2x80x9d, which is a technique utilized in the instant invention. It is an object of this invention to employ this technique to better advantage in a gas analyzer whose sampling is done xe2x80x9cin-situxe2x80x9d, meaning without removal of a sample from the stream, but instead securing data as the consequence of measurements or observations of spectral energy which has been subjected to interaction with the gas stream itselfxe2x80x94either by having passed through the gas stream or by having emanated from it.
Gas filter correlation is a well-known procedure which does not require description here for an understanding of the invention. A useful reference on this subject is xe2x80x9cAnalytical Methods Applied to Air Pollution Measurementsxe2x80x9d by Stevens and Herget, Chapter 10, pages 193-231, published by Ann Arbor Science, 1974, which is incorporated by reference herein for its showing of the applicable theory.
This technology, and the instrumentation provided by this invention, are not limited to applications which are sensitive to stack gases, or even only to actively flowing streams of gases. While such applications represent a very large market, there is a growing need to be aware of conditions in what may suitably be called a xe2x80x9cbulkxe2x80x9d presence of gases. Detection of pollutants and toxic gases in the atmosphere is another example, and an extension of this additional application is surveillance and warning of the presence of undesirable compounds or concentrations of them.
Enclosure and barrier surveillance represents a substantial potential application for this invention. For example, it is useful to know whether a landfill, dump, or even a chemical plant is emitting any specific gas or pollutant. In turn, it may be desired only to know the total emission in all directions, in which event a perimeter would be monitored, or in some specific direction in which a barrier would be monitored. In these situations, there is a xe2x80x9cstreamxe2x80x9d of gases being monitored, although not precisely in the sense of a stack gas in which there is a rapid steady flow. Even so, the concepts of this invention are useful to both, and the term xe2x80x9cstreamxe2x80x9d of gases defines both of them.
Gas filter correlation techniques generally utilize narrow band pass filters. In many applications of this invention, it is quite convenient to use for filters, cells containing specific gases at known and precise concentrations and pressures. These techniques are most suitable for detection and analysis of gases whose spectral absorption pattern includes a number of lines in the band of interest, and in which the xe2x80x9cinterleavedxe2x80x9d regions are also utilized in the procedures. Such gases include carbon monoxide and hydrochloric acid.
More classical techniques are used when instead of many absorption lines within the band of interest, there is merely a wide absorption line or band. Then optical notch filters will be employed instead. Examples of such gas are hydrocarbons and sulfur dioxide.
The apparatus of this system can utilize either optical filters or gas cells, and the generic terms xe2x80x9cfilter meansxe2x80x9d and xe2x80x9cfiltersxe2x80x9d is used for both of them. In addition, the sensitivity of the instrument can be improved by providing a narrow band pass filter that limits the energy reaching the detector to those wavelengths that are of interest.
Also, while the measurement of concentration of a selected gas may be of primary interest in many installations, in others the detection of the presence of that gas may be of primary concern, therefore this invention is not intended to be limited to use with measurement devices, but also extends to surveillance and detection devices where the presence or absence of the compound is of interest.
The United States Environmental Protection Agency has a requirement to audit analyzers with reference cylinder gases for zero and span calibration. In most cases, for in-situ analyzers, this has been done by removing the analyzer from the stack to perform the calibration or audit. Removing the analyzer from the stack for system calibration is a very cumbersome, very time consuming, and annoying process.
It is therefore an object of the present invention to improve an analyzer by providing a method and apparatus for periodic zero and span calibration that does not require removal of the analyzer from the stack. This method involves the use of an audit cell as an internal in-line flow calibration cell. The use of the audit cell allows the present invention to meet Environmental Protection Agency requirements in a very simple way, with no-human interaction.
High and low frequency fluctuation in the infra-red (IR) emission from a source is a serious cause of instrument noise and drift. Previous efforts have relied on a photodiode and feedback to regulate the IR source. Such a system is disclosed and described in U.S. Pat. No. 5,457,320 of R. D. Eckles et al issued Oct. 10, 1995. The system disclosed in this patent merely controls visible emissions from the source, not the IR. Broad band IR emitters such as glow bars and hot filaments, have 16 emission profiles (i.e., photon density wavelength) similar to the ideal xe2x80x9cblack bodyxe2x80x9d. Reducing the power to a device increases the amount of long wave IR radiation and decreases the amount of visible xe2x80x9chigher energy radiationxe2x80x9d. At the same time the amplitude of the entire emission pattern decreases. Therefore there are two competing effects at work which can cause complications when regulating the source using a visible detector such as a photodiode especially for high-precision work.
It is another object of the present invention to incorporate two devices to measure the energy emitted by an IR source to give a direct reading of radiation used in the measurement of the gas and by use of a feedback loop control the source output. The new system uses a narrow band optical filter and a thermopile to measure photon density and regulate the output of an IR source with a feedback loop that is more efficient than previous sources to allow precision work to be carried out with the instrument.
It is still another object of the present invention to provide an audit cell for calibration across a gas stream, without requiring the analyzer to be removed from the stack, that uses isotopically labeled gases to extend the analyzer dynamic range, simplify calibration, and reduce interference. Since the instrument can never duplicate the gas stream optical path length internally in the analyzer an audit cell having a shorter optical path and higher concentration of preference gas so that the product of concentration times the optical path length is identical in the audit cell and the gas stream of the stack works for most gases. For very large stacks it may be impossible to have a concentration sufficient for calibration. For this purpose a calibration xe2x80x9ctrickxe2x80x9d of measuring stable isotopically labeled gases in the path is used. These gases also have a distinctive and measurable absorption (or emission) spectra. Thus isotopically labeled gases at much lower concentration that may be as low as 1% of the principal gas concentration can be used to calibrate the instrument.
Personal computers and personal computer architecture have becomelextremely popular in both general purpose and dedicated applications however their use is limited due to the large size of the motherboard and expansion cards. Thus a specification, both mechanical and electrical, has been conceived to optimize computers for use where the large size personal computer option is not viable. This specification is known in the art as PC/104 architecture. By using this architecture the form factor or dimensions of the computer have been reduced to less than four inches square and eliminates the means for back planes, card cages, by using a self-stacking bus. The system also minimizes component power consumption by reducing required bus drive power of most signals.
It is therefore another object of the present invention to adapt PC/104 bus architecture to allow an onboard computers for instrument control. In applications where the onboard PC/104 architecture is not possible, the computer will be remote and the stack-mounted analyzer will consist of an optical head with a series of serial modules used to send signals to a remote computer. A PCB may be used at the stack-mounted analyzer to allow the use of Ethernet instead of serial communication. Either way, digital signals will be sent to a remote PC.
Still another object of the present invention is to provide onboard electronics and monitoring system in the form of a computer having a PC/104 form factor. This allows the use of an onboard computer in the analyzer that conforms to the PC/104 specification including both mechanical and electrical specifications and permits a highly rugged and miniature computer to be used on- board the in-situ gas analyzer.
When the term xe2x80x9cgasxe2x80x9d is used herein, relating to the substance being detected or measured, it is not intended to be limited to compounds in their gaseous state. The measurement or detection of opacity is also comprehended, and this may involve the detection and measurements of particulate conveyed in a gas stream. Such a situation is also intended to be included in the term xe2x80x9cgasxe2x80x9d.
It is an object of this invention to provide a system which can have a direct zero and span measurement, even with gases flowing or present in the apparatus; which can readily and automatically be calibrated, and all interferences automatically rejected; which can be constructed so as readily to be accessed for routine repair and maintenance, and even deposed at a considerable distance from the situs being sampled or observed; which is sufficiently heat resistant that its readings do not stray during temperature excursions; which rejects spurious signals from its surroundings; and which is forgiving of substantial physical shifts and changes in the physical environment, such as by dimensional expansion and contraction.
Still further objects are to provide better techniques for internal calibration of the instrument, for more efficient optical path, and for decreased sensitivity to external physical distortions such as vibratory and temperature induced dimensional shifts.
Apparatus according to this invention utilizes spectral energy which has been subjected to interaction with a gas either by having passed through the gas, or by having emanated from it and uses a unique internal audit cell for system calibration.
The present invention is an improvement to the invention disclosed and described in U.S. Pat. No. 4,632,563 issued Dec. 30, 1986 and U.S. Pat. No. 4,746,218 issued May 24, 1988 to Harry C. Lord.
The heart of this invention is a new and improved analyzer with an array of filter means for reference and optionally for calibration, to which a beam of spectral energy is directed. The beam will, before or after interaction with these filters, also interact with the gas, either by being passed through the gas, or by having emanated from it. A detector is responsive to the energy which has interacted both with the gas and with the reference filters (optionally also with the calibration filters).
In one embodiment, the analyzer operates within itself to direct the energy to be analyzed to selected filters, but can be placed anywhere that it receives an incoming beam segment, which can be fixed, or where it can produce a beam to be passed to the gas, which beam can also be fixed. Optional means can be provided to present different filters to the beam from time to time.
Optical devices can be placed in the path of the beam at appropriate locations to exert a focusing action which assures that regardless of physical shifts or movements of reasonable magnitude, the beam will fully fall into-the face of the detector. In other portions of the system, a split-aperture Cassegrainean telescope which both projects the beam and receives the returned beam, or a lens system, or cube corner reflectors can be provided which also reduce sensitivity to dimensional variations.
In one application, a spectral source provides a beam which is passed twice through a stream of the gas (being reflected after the first pass). Alternatively, the source for one of the fixed beams may be emissions from the process or from the gases themselves.
In another application, the spectral beam may be passed a single time through the stack, and then received and treated by the analyzer. This embodiment may also be adapted to receive and treat a beam of energy derived directly from the gas itself, by emission, or by xe2x80x9cobservingxe2x80x9d the process itself, such as by receiving energy from a process flame in a burner, or from the gaseous region above a process, such as just above the molten glass surface in a glass furnace.
In still another applications, the beam path traverses a boundary or a barrier just above the ground. This enables a detection or surveillance type operation.
Generally, infra-red energy will be utilized with this invention. Gases of frequent concern have useful absorption patterns in the infra-red region. Furthermore, infra-red radiation can conveniently be emitted or collected. However, visible and ultra-violet energy may also be used advantageously in some applications. The invention is not intended to be limited to one in which only infra-red radiation is utilized. Of course, filters respective to the wavelengths being employed will be employed in place of these which are respective to infra-red radiation.
When reference cells are used for filters, they can contain mixed gases to measure parameters of more than one gas, whose pertinent spectra do not interfere with one another. Carbon monoxide and sulfur dioxide constitute one such mixture.
According to yet another preferred but optional feature of the invention, a chopper is placed in the energy path, whereby to provide pulses of energy to the detector at a frequency determined by the chopper, thereby providing means to reject spurious data.
According to yet another preferred but optional feature of this invention, a separate calibration beam path is provided which by-passes the stream on its way to the analyzer in order to give a zero-based reading.
According to-still another preferred but optional feature of the invention, a pair of cube-corner retro-reflectors are provided to return the beam, one on each side of the gas stream, one to return the beam across the stream, and the other to return it in the calibration mode without crossing the gas stream.
According to yet another preferred but optional feature of the invention, gas cells used for calibration have two separate gas chambers containing gases at different concentrations and pressures in order to provide two sets of data for the solution of two simultaneous equations.
In still another preferred but optional feature of the invention an audit cell is provided for in-situ zero system calibration. The use of an audit cell permits the calculation of an accurate zero and span calibration points. An incremental additions method is used in conjunction with the audit cell for modeling the instrument zero. The preference gas is added to the audit cell in a concentration that has a pre-determined ratio to the gas being analyzed and the optical path length employed in the measurement. The audit cell is placed in series with the instrument and concentrations of a reference gas are incrementally added. By knowing what the concentration of the reference gas is inside the audit cell, a zero can be retroactively calculated that is much more accurate than what is normally generated by using a zero mirror in the instrument, which includes correction for any interferences from other interfering gases present in the gas stream.
Also in another preferred embodiment of the invention the reference gas is isotopically labeled with a lower abundance stable naturally occurring isotope of one of the elements of the gas; e.g. 18O instead of 16O to allow calibration where a direct ratio of reference gas to the stack gas is not possible. Use of an isotopically labeled preference gas can reduce the amount of concentration needed to 1% times the concentration ratio in some cases, or less.
The use of the audit cell allows the duplication of the optical path length internally in the analyzer by using the higher concentration of the preference gas so that the product of the concentration times the optical path length is identical in the audit cell and in the stack. This method works for most gases and for all gases that will be measured in the ppm or ppb levels.
However, for measuring and calibrating a gas at 10% like carbon dioxide (CO2) for example in a stack that is 30 foot wide, in a double-pass system the optical path length is twice the stack diameter which is 60 feet. Thus, you would require 600% feet. If you have a one foot audit cell, you would have to have 600% of the preference gas in the audit cell which is impossible. It was recognized that oxygen has suitable isotopes, for example 18O, and carbon has a suitable isotope, for example 13C, each of which is only a small percent of the primary naturally occurring isotope(s). Furthermore the absorption spectra for the molecule with the different isotope(s) is unique and hence may be used for the measurement. When you measure the 10% Co2 if the instrument looks specifically at 13CO2, the concentration is now only 1% times the inverse ratio of optical path length to cell length and now it is at 6% feet and the audit cell can now be easily filled with calibration gas. Thus, by isotopically labeling reference gases in the audit cell, the system can perform a calibration trick using a much lower concentration.
Another important aspect and optional feature and improvement in the invention is the use of a feedback loop for an IR source control. The improved system described herein uses two devices to measure energy emitted by the IR source. A narrow band pass optical filter (NBOF) to isolate a specific spectrum window and the thermopile to measure the photon density of radiation. The advantage of this configuration is that the NBOF can be used to isolate the same spectral window as the one in the measurement of the gas by the instrument (e.g., 2,100xc2x125 cmxe2x88x921 for carbon monoxide). With this approach the thermopile gives a direct reading of radiation used in the measurement of gas rather than an indirect measurement that might be obtained by use of a visible radiation detector such as a photodiode. This regulation of the IR source using the analyzer computer and a feedback loop from a narrow band optical filter and thermopile is more efficient than previous versions thereby allowing ultra-high precision work to be carried out with the instrument.
Still another optional feature and improvement of the analyzer disclosed herein is the use of improved instrument configuration such as an on-board computer. The on-board configuration for the computer or PC for application where the process and environment and conditions allow is a PC/104 architecture or form factor. The PC/104 form factor is highly rugged and miniaturized, since it has no back plane board and yet has not been presently used for industrial in-situ gas analyzers. The use of the PC/104 form factor permits the PC to be incorporated into the analyzer because it has a configuration that can be fit in a space of less than four inches square.
The analyzer uses an IR detector mounted on a pre-amplifier board which relies on a low noise field effect transistor (FET) operational amplifier to make detector noise a limiting performance factor. A separate feedback loop controls the detector temperature within plus or minus 0.005xc2x0 C. or better.
The system and circuit is also designed to accommodate a variety of detector types for detection of a wide range of spectral wavelengths from below 0.5 xcexcm to above 20 xcexcm with increased sensitivity. Little or no change to the signal processing electronics is required for swapping detectors making this a powerful modular system. The system described allows important environmental gases (e.g., H2SO4, SO3, NH3) to be measured reliably in an industrial environment for the first time. A signal processing board is mounted on a chassis known as a PC/104 stack and receives a signal from the pre-amplifier board. Temperature and pressure control loops are managed by an additional board mounted on the PC/104 stack. Files can be accessed and transferred remotely using off-the-shelf remote access software.
The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: